For some of us, making our smile bright and shiny is just a part of our morning ritual. For movie stars like Elizabeth Banks, a five-digit payday is on the line. But the Hunger Games star isn't pocketing the cash — she's earning it to help thousands of children nationwide.

In this exclusive interview, the first thing we at Wetpaint Entertainment want to know is whether Elizabeth ever thought there'd be an opportunity to earn a $21K charitable donation just by gargling mouthwash.

"No, but I'm really excited that there is," she laughs. "It was one of those super no-brainer, easy ideas, right? First of all, oral health is just really important. There are 130 million Americans who lack dental insurance in this country, and tooth pain is one of the leading causes of kids missing out on school."

And the 40-year-old actress will enjoy the benefits, too: "I make my living with my mouth. It's really important to me that it look good but equally important that it be healthy. And that's where Listerine comes in, and the 21 Day Challenge is just a really easy way to make sure that I essentially clean virtually 100 percent of my mouth."

Plus, she tells us that we can all help raise money for Oral Health America's program. "21 days is the amount of time that it takes to form a healthy habit, which is why it's the 21 Day Challenge — swishing twice a day for 21 days," she explains. "Easy, breezy, and anybody can join me in this challenge by posting a picture of themselves swishing — or a Swish Selfie, as we like to call it here — to Listerine's Facebook page, and then they're going to donate five bucks for every picture that gets uploaded."